In this paper, recent progress of phase change memory (PCM) is reviewed. The electrical and thermal properties of phase change materials are surveyed with a focus on the scalability of the materials and their impact on device design. Innovations in the device structure, memory cell selector, and strategies for achieving multibit operation and 3-D, multilayer high-density memory arrays are described. The scaling properties of PCM are illustrated with recent experimental results using special device test structures and novel material synthesis. Factors affecting the reliability of PCM are discussed.

Phase Change Memory

Nonvolatile RAM using resistance contrast in phase-change materials [or phase-change RAM (PCRAM)] is a promising technology for future storage-class memory. However, such a technology can succeed only if it can scale smaller in size, given the increasingly tiny memory cells that are projected for future technology nodes (i.e., generations). We first discuss the critical aspects that may affect the scaling of PCRAM, including materials properties, power consumption during programming and read operations, thermal cross-talk between memory cells, and failure mechanisms. We then discuss experiments that directly address the scaling properties of the phase-change materials themselves, including studies of phase transitions in both nanoparticles and ultrathin films as a function of particle size and film thickness. This work in materials directly motivated the successful creation of a series of prototype PCRAM devices, which have been fabricated and tested at phase-change material cross-sections with extremely small dimensions as low as 3 nm × 20 nm. These device measurements provide a clear demonstration of the excellent scaling potential offered by this technology, and they are also consistent with the scaling behavior predicted by extensive device simulations. Finally, we discuss issues of device integration and cell design, manufacturability, and reliability.

Phase-change random access memory: a scalable technology

A phase-change memory element with side-wall contacts is disclosed, which has a bottom electrode. A non-metallic layer is formed on the electrode, exposing the periphery of the top surface of the electrode. A first electrical contact is on the non-metallic layer to connect the electrode. A dielectric layer is on and covering the first electrical contact. A second electrical contact is on the dielectric layer. An opening is to pass through the second electrical contact, the dielectric layer, and the first electrical contact and preferably separated from the electrode by the non-metallic layer. A phase-change material is to occupy one portion of the opening, wherein the first and second electrical contacts interface the phase-change material at the side-walls of the phase-change material. A second non-metallic layer may be formed on the second electrical contact. A top electrode contacts the top surface of the outstanding terminal of the second electrical contact.

Phase-change memory

The global demand for data storage and processing has increased exponentially in recent decades. To respond to this demand, research efforts have been devoted to the development of non-volatile memory and neuro-inspired computing technologies. Chalcogenide phase-change materials (PCMs) are leading candidates for such applications, and they have become technologically mature with recently released competitive products. In this Review, we focus on the mechanisms of the crystallization dynamics of PCMs by discussing structural and kinetic experiments, as well as ab initio atomistic modelling and materials design. Based on the knowledge at the atomistic level, we depict routes to improve the parameters of phase-change devices for universal memory. Moreover, we discuss the role of crystallization in enabling neuro-inspired computing using PCMs. Finally, we present an outlook for future opportunities of PCMs, including all-photonic memories and processors, flexible displays with nanopixel resolution and nanoscale switches and controllers. Chalcogenide phase-change materials (PCMs) are leading candidates for non-volatile memory and neuro-inspired computing devices. This Review focuses on the crystallization mechanisms of PCMs as well as the design principles to achieve PCMs with high switching speeds and good data retention, which will aid the development of PCM-based universal memory and neuro-inspired devices.

Designing crystallization in phase-change materials for universal memory and neuro-inspired computing

Phase change memory (PCM) is an emerging technology that combines the unique properties of phase change materials with the potential for novel memory devices, which can help lead to new computer architectures. Phase change materials store information in their amorphous and crystalline phases, which can be reversibly switched by the application of an external voltage. This article describes the advantages and challenges of PCM. The physical properties of phase change materials that enable data storage are described, and our current knowledge of the phase change processes is summarized. Various designs of PCM devices with their respective advantages and integration challenges are presented. The scaling limits of PCM are addressed, and its performance is compared to competing existing and emerging memory technologies. Finally, potential new applications of phase change devices such as neuromorphic computing and phase change logic are outlined.

http://poplab.stanford.edu/pdfs/Raoux-PCMreview-mrsbull14.pdf

Phase change materials and phase change memory

A phase change memory device includes a switch and a storage node connected to the switch. The storage node includes a first electrode, a phase change layer and a second electrode. The phase change layer is formed of an InSbTe compound doped with Ge. In a method of operating a phase change memory including a switch and a storage node, the switch is maintained in an on state, and a first current is applied to the storage node.

Phase change materials, phase change random access memories having the same and methods of operating phase change random access memories

Phase change random access memory (PRAM) is unique among semiconductor devices because heat is intrinsic to the operation of the device, not just a by-product. Here, we apply a material that is exotic in the context of typical semiconductor devices but has highly desirable properties for PRAM. Thin films of C60 are semiconducting and show very low thermal conductance. By inserting a C60 layer between the phase change material and the metal electrode, we dramatically reduced the heat dissipation and, thereby, the operating current. A PRAM device incorporating a C60 layer operated stably for more than 105cycles.

Fullerene thermal insulation for phase change memory

Nanometer scale Ge2Sb2Te5 (GST) domains formed by immiscible mixture of GST-SiOx at room temperature and 180 °C show remarkable suppression in electrical and thermal conductivity. Thermal boundary resistance with increased GST-SiOx interface becomes crucial to the reduction in thermal conductivity. These conductivity reductions concurrently result in the reduction in programming current and power consumption in phase change memory devices.

Low thermal conductivity in Ge2Sb2Te5–SiOx for phase change memory devices

Electrical characteristics of Ge2Sb2Te5 (GST) nanoparticles have been examined for a phase-change memory applications. The GST nanoparticles were generated by in situ pulsed laser ablation and their crystal structure formation was confirmed [H. R. Yoon et al., J. Non-Cryst. Solids 351, 3430 (2005)]. A stacked structure of the GST nanoparticles with 10nm of average diameter shows reversible nonvolatile switching characteristics between a high resistance state and a low resistance state as in the phase-change memory consisting of bulk GST thin film. Experimental results indicate that it is highly probable to test scaling issues of the phase-change memory with well-defined GST nanoparticles.

Nonvolatile switching characteristics of laser-ablated Ge2Sb2Te5 nanoparticles for phase-change memory applications

There is disclosed a method for making array capable of realizing large output and large-scale integration by using a heterogenous film which can electrically insulate between LEDs, (LEDs) by the diffusion of an impurity into a substrate. The improved LED array manufacturing method includes the steps of: forming a luminescent layer, of a first conductivity type, a transparent layer and a cap-layer over the semiconductor substrate, forming of a cap layer made into a given pattern by etching a given portion of said cap layer; forming of a diffusion region converted into the first conductivity type by the injection of an impurity into a given portion of the transparent layer; forming an oxide film on the entire surface except the area where the cap layer is covering the transparent layer 13; and forming an electrode over the cap layer and a common electrode under the semiconductor substrate.

Ki-Joon Kim

Papers

Phase change materials, phase change random access memories having the same and methods of operating phase change random access memories

Fullerene thermal insulation for phase change memory

Low thermal conductivity in Ge2Sb2Te5–SiOx for phase change memory devices

Nonvolatile switching characteristics of laser-ablated Ge2Sb2Te5 nanoparticles for phase-change memory applications

Method for making an LED array